Plus a couple of related ActionPack patches.

Regexp#match? should be considered to be part of the Ruby core library. We are
emulating it for < 2.4, but not having to require the extension is part of the
illusion of the emulation.

Style/SpaceBeforeBlockBraces
Style/SpaceInsideBlockBraces
Style/SpaceInsideHashLiteralBraces

Fix all violations in the repository.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

Where appropriate, prefer the more concise Regexp#match?,
String#include?, String#start_with?, or String#end_with?

CVE-2016-0752

While trying to provide a reproducible test for #17008 I stumbled on
this one. Seems to be quite an old piece of code, but its definitely
useful in situations like the reproducible test cases like the one
above.

By default, variants in the templates will be picked up if a variant is set
and there's a match. The format will be:

  app/views/projects/show.html.erb
  app/views/projects/show.html+tablet.erb
  app/views/projects/show.html+phone.erb

If request.variant = :tablet is set, we'll automatically be rendering the
html+tablet template.

In the controller, we can also tailer to the variants with this syntax:

  class ProjectsController < ActionController::Base
    def show
      respond_to do |format|
        format.html do |html|
          @stars = @project.stars

          html.tablet { @notifications = @project.notifications }
          html.phone  { @chat_heads    = @project.chat_heads }
        end

        format.js
        format.atom
      end
    end
  end

The variant itself is nil by default, but can be set in before filters, like
so:

  class ApplicationController < ActionController::Base
    before_action do
      if request.user_agent =~ /iPad/
        request.variant = :tablet
      end
    end
  end

This is modeled loosely on custom mime types, but it's specifically not
intended to be used together. If you're going to make a custom mime type,
you don't need a variant. Variants are for variations on a single mime
types.

Before this patch, every request in development caused the template
to be compiled, regardless if it was updated in the filesystem or not.
This patch now checks the timestamp and only compiles it again if
any change was done.

While this probably won't show any difference for current setups,
but it will be useful for asset template handlers (like SASS), as
compiling their templates is slower than ERb, Haml, etc.

[#4523 state:resolved]
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

[#4522 state:resolved]
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

Improve performance of the rendering stack by freezing formats as a sign that they shouldn't be further modified.

  * request.formats is much simpler now
    * For XHRs or Accept headers with a single item, we use the Accept header
    * For other requests, we use params[:format] or fallback to HTML
    * This is primarily to work around the fact that browsers provide completely
      broken Accept headers, so we have to whitelist the few cases we can
      specifically isolate and treat other requests as coming from the browser
    * For APIs, we can support single-item Accept headers, which disambiguates
      from the browsers
  * Requests to an action that only has an XML template from the browser will
    no longer find the template. This worked previously because most browsers
    provide a catch-all */*, but this was mostly accidental behavior. If you
    want to serve XML, either use the :xml format in links, or explicitly
    specify the XML template: render "template.xml".